Treatment of hydrocarbon gases



Marh 16, 1943.

H. Y. HYDE ETAL `TREATMENT OF HYDROCARBON. GASES Filed May 10, 1940 PUR/F/ED PROPANE LIGHTER FRACT/ON HfRJcH/SL K HYDE CLARE K. l//LA/vo L/o/VEL 6. 6,4 6714 UN A ATTO/@var Patented Mar. 16, 1943 UNITED y STATES PTENT OFFICE TREATMENT F HYDROCARBON GASES Herschel Y. Hyde, Associated, Clare Kenneth Vilaml, --Martinea and Lionel S. Galstaun, San

Francisco, Ualitrassignors to Tide Water Associated 0il-Con1panY, San Francisco, Calif., a-

corporation of Delaware Application May 10, 1940, Serial No. 334,408

12 Claims. '(Cl. 260676) This invention relates'to the separation of hydrogen sulphide from light hydrocarbon gasesy crude oils, the content of HaS in the Ca-C4 frac- 'of HzS per 100 standard cubic feet of gas. With Another object is to provide a means whereby such hydrogen. sulphide removal may be conducted with satisfactory regeneration of 'an absorbing agent.

Still another object is to efficiently prepare a mixture of Ca and Ci hydrocarbons suitable tor polymerization purposes.

In the manufacture of gasoline motor fuel by the so-called cracking processes, wherein high boiling hydrocarbons are converted into lower boiling hydrocarbons by the application of high temperatures, with or without pressure, there are formed large quantities of normally gaseous hydrocarbons (containing one, two, three, and

four carbon atoms to the molecule and commonly referred to as C1, C3, Ca, and C4 hydrocarbons respectively). Most of these have too high a vapor pressure to be included in marketable gaso- Y line. However, the C4 hydrocarbons have suflciently low vapor pressures that a certain percentage may be incorporated in the finished ga 1oline without raising the vapor pressure of the finished product above thedesired value.

Inaddition to the light hydrocarbons formed by cracking processes, sulphur compounds present become desirable to segregate a fraction predominating in C3 and C4 hydrocarbons for charge material to these processes. When such a fraction is segregated by distillation from cracking still gases it will, contain the major portion of the H28 togther'with some mercaptans especially certain high sulphur crudes this fraction may contain as high as 30% by volume of. HzS. The presenceA of HzS and mercaptans is detrimental to polymerization, alkylation, and such synthetic processes in that it causes corrosion of equipment, poisoning of catalysts, and contamination of products.

The removal ot the hydrogen sulphide and mercaptans may be eiiected by washing the Ca-C4 fraction with caustic soda solution, but this entails the consumption of large amounts of caustic soda due to the large quantity of hydrogen sulphide present.

To prevent the consumption or chemicals, and also to allow the recovery of hydrogen sulphide as a by-product, regenerative puricazion processes are commonly employed. According to these regenerative processes the Ca--C4 lfraction is washed with an alkaline solution which is capable of absorbing hydrogen sulphide by a reversible reactic .1. After the alkaline solution is saturated, or nearly so, with hydrogen sulphide it is passed to a still wherein the hydrogen sulphide is boiled oil, and recovered if desired, and the regenerated alkaline solution returned to the washing cycle. Various alkaline solutions are suitable for this purposesuch as aliphatic amines, mono, di, and. tri-ethanolamines, diaminopropanol, sodium phenolate. tripotassium phosphate, etc.

However, it has been found that, at times, due apparently to the type of cracking stocks used or to conditions obtained during cracking, the

C3C4 fraction contains in addition to the HzS and mercaptans small amounts of aliphatic acids. Acetic acid has been identied in this fraction, and the presence of other homologues is indicated. Although these aliphatic acids are at times present in this fraction in only' very small concentration, suchas in the order of 0.01% by weight, they are considerably stronger acids than the HzS and unite with the alkaline absorptive solution to form comparatively stable compounds which are not satisfactorily removed in the regenerative stage. Consequently, there is a gradual accumulation of aliphatic acids in the ab-l methyl mercaptan. When cracking sulphur bearing oils, such/as residuum -from typical California sorptive solution during repeated use. Besides consuming the chemical and thus lowering the eiiiciency for hydrogen sulphide removal, the accumulation of these aliphatic acids creates a tendency o f the alkaline solution to foam in the absorber and thereby often so greatly interferes with the throughput capacity oi the plant that the reagent 'must be entirely replaced at agreat increase in cost of operation.

It is thus seen to be desirable to remove, or at least reduce the concentration of, the aliphatic acidsin the C3--C4 fraction prior to removal of the H2S by the regenerative process. While this may be accomplished by washing the fraction with water, such procedure requires the disposal of rather large quantities of waste wash water saturated with hydrogen sulphide. Washing the fraction with caustic soda, or sodium sulphide, solution likewise is partially effective 'in removing the aliphatic acids, but the caustic soda, or sodium sulphide, solution very soon becomes ineffective and requires frequent replacing.v

According to the present invention a means is provided whereby the H28 may be removed by the regenerative process without contamination of the absorptve alkaline solution with the aliphatic acids. To this end the Ca-C4 fraction, containing hydrogen sulphide and aliphatic acids is frac.. tionally distilled into two portions the lighter of which will contain the C3 hydrocarbons, the hydrogen sulphide, any lighter hydrocarbons, and substantially no C4 hydrocarbons. The heavier portion will contain the C4 hydrocarbons, any heavier hydrocarbons, and substantially no C3 hydrocarbons or hydrogen sulphide. When such a split is made between the C: and C4 hydrocarbons, it will be found that the heavier portion will contain the b ulk of the mercaptans, chiefly methyl mercaptan. Likewise, the aliphatic acids will be concentrated in the heavier portion.

The lighter portion containing the C3 hydrocarbons and the hydrogen sulphide is subjected to the regenerative H28 removal described above, and, since the aliphatic acids are sufficiently absent, the separation of the H25 and the regeneration of' the alkaline solution proceeds in an orderly manner.

The heavier portion containing the C4 hydrocarbons, the mercaptans and the aliphatic acids I is washed with an alkali solution, such as aque ous NaOH for example. Any suitable strength of alkali may be used, 20% NaOH being preferred. Since the quantity of aliphatic acids and mercaptans is very much less than the quantity of HzS in the Ca-C4 fraction, the consumption of alkali in treating the heavy portion is not excessive as would be the case 4if the entire Ca-C4 fraction were treated with NaOH. However, if the mercaptan content of the heavy portion is high, advantageous results may be obtained by treating the heavy portion according to the process of U. S. Patent Number 2,183,968 issued December 19, 1939, to McCormick and Lazar, according to which the aliphatic acids are rst removed in a. non-regenerative stage with NaOH and then the mercaptans are removed in a second stage with an alkali which may be regenerated and returned for further treatment. For such second stage any alkali capable of absorbing mercaptans at low temperatures and releasing them at high temperatures may be used. Aqueous caustic soda, mixtures of potassium hydroxide e and potassium isobutyrate, etc., may be used.

Having removed the HzS from the C3 portion and the mercaptans from the C4 portion, the two portions are then blended together to form a puried C3-C4 fraction suitable as charge material for polymerization, alkylation, and other synthetic processes.

In the attached drawing there is shown for the purposes of illustration an application of the stabilization of untreated cracked gasoline and comprised largely of C3 and C4 hydrocarbons-together with hydrogen sulphide and also traces of acetic acid and/or other aliphatic acids is charged to the purification system for removal of hydrogen sulphide to prepare the gas for subsequent manufacturing processes.

The contaminated gas in line I is charged under suitable pressure to a fractionating tower 2 where the gas is fractionated into two fractions according to boiling. range. The overhead product comprising the propane and lower boiling constituents is removed in line 3 and charged to absorber 4 for removal of hydrogen sulphide and other acidic gases'contained in the fraction. A suitable solution capable of removing hydrogen sulphide and other acidic constituents from the gas fraction is charged from tank through line 6 into absorber 4. The preferred solution for this purpose is a 20% solution of diethanolamine in water, although other concentrations of the amine either stronger or weaker than 20% are satisfactory, or any other chemical capable of removing hydrogen sulphide and other acidic constituents by means of a reversible reaction may be employed. Ordinarily the reaction in tower 4 is conducted with the hydrocarbons in the vapor phase, and under a pressure approximating that in fractionating tower 2, which may preferably be about 225 pounds per square inch, gage, although other operating conditions may be employed if desired.

The hydrogen sulphide together with any carbon dioxide present is absorbed from the gas into the amine solution at a temperature of 100-150 F., and the foulamine solution is passed from the `absorber' through control valve 1 and line 8 into the solution regenerating still 9. Heat is applied to the solution in still 9, by reboiler I0 or other suitable means, to reverse the reaction, and the hydrogen sulphide in relatively pure form leaves still 9 through the overhead line II. The hydrogen sulphide product at this point is generally -99% pure hydrogen sulphide with traces of hydrocarbons and carbon dioxide, the latter being removed along with the hydrogen sulphide. The regenerated amine solution is pumped from still 9 through line I2, cooler I 3, and line 6 back into absorber 4 for further use. The purified propaneand-lightergas fraction leaves absorber 4 through line I4 and may contain 20-50 grains hydrogen sulphide per cubic feet, or higher or lower depending on the eiliciency of the treatment.

The butane-and-heavier residue fraction from `fractionating tower 2 is withdrawn as a liquid under pressure through line I 5 through cooler I6 and passed through caustic contact tower I'I where mercaptans and the aliphatic acids are removed from the fraction by reaction or solution in caustic soda or other alkali supplied through line I8. Tower I1 may, if desired, contain contact material such as Raschig rings, baffles, etc., or, alternately, any well known contact apparatus may be substituted for tower I1. From time to time it will be necessary to withdraw the spent caustic solution through line IS and inject new caustic into the tower through line I8. The purified butane-andsheavier fraction leaves the caustic tower I1 through line 20, valve 2 I, and line 36.

I1' desired, particularly when the quantity of mercaptans is high, the concentration of fresh caustic in tower I1 may be maintained suiliciently low that only the aliphatic acids and small amounts of mercaptans are removed. The hyinventlon. A hydrocarbon gas derived from the 75 dIOCarbOnS then leaving Contactor I1 will con- D l tain the major portion of the mercaptans'present.

By' closing valve 2| and opening valves 22 and 23,

' the mercaptan-containing "hydrocarbons are passed through line 24 into contactor 25. A caustic soda, or other alkaline, solution is supplied to contacter 25 from tank 26, through line 21. In

contacter 25 the mercaptans are removed by the` caustic solution and the purified hydrocarbons pass out through lines 28 and 35. The foul alkaline solution is passed from'contactor 25 through control valve 29 and line 3il into regenerating still 3l. Heat is applied'to the solution in still 3l by reboiler 32 and the mereaptans in relatively pure form leave still 3| through line 33. 'I'he regenerated alkaline solution is pumped from still 3l fraction in line 36 is withdrawn through valve 33 and is mixed with the propane-and-lighter fraction in line 39. Depending on the use to be made of the puried Cx-Ci mixture,'the mixture in line 39 may be liquefied in condenser 40 and passed to storage tank 4l. Ca--C4 mixture is desired, condenser 40 may be operated as a vaporizer and the mixture of liquid an'd gas in line 39 completely vaporized.

In the application of the invention described above and illustrated in the drawing the treatment of the light fraction was shown in the gaseous state and the treatment of the heavy fraction in the liquid state. By proper choice of tempera- -tures and pressures either or both of these fractions may be treated in either liquid or gaseous state, as desired.

By means of the invention it has been found possible to'process greatly increased volumes of gas,` containing from 0.001% to 0.1% by Weight of aliphatic acids, for purification with greatly decreased chemical consumption in the regenerative process, While at the same time eliminating foaming and frothing difculties which are usually encountered when the Whole gas is processed in a single system. It has also been found that when purifying refinery gas according to the invention, that the hydrogen sulphide product is of a higher purity than formerly obtained under conventional solutions which are continuously regenerated Alternatively, if a gaseous operation where the total gas is processed inthe absorber, since the hydrogen sulphide product under the invention contains substantially no mercaptans and also decreases quantities of other constituents such as hydrocarbon gases.

In referring herein to the C3 and C4 fractions,

it will be understood that such fractions include both saturated and unsaturated hydrocarbons, normally existing in gaseous form, which constitute the bulk of the charge necessary for later use in the desired processes, such .as polymerization, alkylation, or other processes involving chemical reactions wherein relatively great purity is necessary, because the presence of the now separately removed impurities not only tends to disturb such chemical reactions but produces an impure product. I

Further, the separate removal of impurities greatly enhances operating facilities by preventing the foaming of the alkaline solution during absorption and distillation and thus the mel chanical carrying over of impurities with the gas to be processed is eliminated as well as corrosion and other deterioration,

with its concomitant shut downs,v such as will be due to the presence of acids and sulphur compounds.

It will be recognized that, from the practical or commercial refinery standpoint, the process is primarily a continuous process in which a continuous stream of gases is fractionated and the fractions are continuously treated with alkaline hydrocarbons which comprises: fractionally distilling a mixture of C3 and C4 hydrocarbons containing hydrogen sulphide, mercaptans, and aliphatic acids `to obtain a light fraction comprising said C: hydrocarbons predominantly together with said hydrogen sulphide, and a heavy fraction comprising said C4 hydrocarbons predominantly together withsaid mercaptans and i said aliphatic acids.; ,contacting a continuous stream of said lightv fraction in a contact zone with an alkaline reagent capable of absorbing ms and liberating the same upon heating; removing contacted alkaline reagent from-said contact zone; distilling HzS from said'contacted alkaline reagent to regenerate the same; returning regenerated alkaline reagent to lsaid contact zone; contacting said heavy` fraction with an Aalkali'solution to remove said mercaptans and said aliphatic acids; removing the purified heavy fraction from the alkali solution; and blending the thus treated light and heavy fractions to obtain a. purified Ca--C4 mixture.

2. The process according to claim l in which the `first stated alkaline reagent is an aqueous solution of diethanolamine.

3. The process according to claim 1in which the first stated alkaline reagent is a solution of about 20% diethanolamine in water.

4. The process according to claim 1 in which the percentage of aliphatic acids in the mixture of C: and C4 hydrocarbons before treatment thereof is from 0.001% to 0.1% by weight.

5. In a process wherein a cracked distillate containing C4 hydrocarbons together with lighter and heavier" hydrocarbons. hydrogen sulphide, and aliphatic acids is separated by fractional distillation to yield a fraction comprising' pre dominantly C3 and C4 hydrocarbons together with the hydrogen sulphide and certain aliphatic acids, and the hydrogen sulphide is removed from said fraction by treatment with an alkaline reagent which is regenerated and reused in the process, themethod of preventing contamination of said alkaline reagent by said aliphatic acids remaining in said fraction as a result of said distillation, which comprises: ilrst subjecting said Ca-C4 fraction to fractional distillation to remove therefrom the C: hydrocarbons and prising predominantly C3 and C4 hydrocarbons together with the hydrogen sulphide, certain aliphatic acids, and low boiling mercaptans, and the hydrogen sulphide is removed from said fraction by treatment with an alkaline reagent which is regenerated and reused in the process. the method of preventing contamination of said alkaline reagent by said aliphatic acids remaining in said fraction as a .result of said distillation, which comprises: first subjecting said C3-C4 fraction to fractional distillation to remove therefrom the C3 hydrocarbons and the hydrogen sulphide whereby said aliphatic acids and low boiling mercaptans remain with the C4 hydrocarbons, then treating the C3 fraction with said alkaline reagent to remove the hydrogen sulphide, and removing the mercaptans and aliphatic acids from the'C4 hydrocarbons by treatment with further alkaline solution.

7. The process of purifying normally gaseous hydrocarbons which comprises: fractionally distilling a mixture of C3 and C4 hydrocarbons containing hydrogen sulphide, mercaptans, and aliphatic acids to obtain a light fraction comprising said C3 hydrocarbons predominantly together with said hydrogen sulphide, and a heavy fraction comprising said C4 hydrocarbons predominantly together with said mercaptans and said aliphatic acids; continuously passing a stream of the light fraction through a zone wherein the hydrogen sulphide is abstracted by contact with an alkaline reagent; continuously withdrawing the contacted alkaline reagent from said zone and removing the hydrogen sulphide therefrom; continuously returning the purified reagent to said zone; removing the mercaptans from said heavy fraction with an alkaline reagent; and blending the thus treated light and heavy fractions.

8. The process according to claim 7 in which the alkaline reagent used.to remove the mercaptans from the heavy fraction is aqueous caustic l soda.

9. 'I'he process of purifying normally gaseous hydrocarbons which comprises: fractionally distilling a mixture of C3 and C4 hydrocarbons containing hydrogen sulphide, mercaptans, and aliphatic acids to obtain a light fraction comprising said Ca hydrocarbons predominantly together with said hydrogen sulphide, and a heavy fraction comprising said C4 hydrocarbons predominantly together with said mercaptans and said aliphatic acids; continuously passing a stream of the light fraction through a zone wherein the hydrogen sulphide is abstracted by contact with an alkaline reagent; continuously withdrawing the contacted alkaline reagent from said zone and removing the hydrogen sulphide therefrom; continuously returning the purified reagent to said zone; removing the aliphatic acids from said heavy fraction with an alkaline reagent; continuously passing a stream of the thus purified heavy fraction through a second zone wherein the mercaptans are abstracted by contact with an alkaline reagent; continuously withdrawing the contacted alkaline reagent from said second zone and removing the mercaptans therefrom; continuously returning the thus puried reagent to said second zone; and blending the thus treated light and heavy fractions.

10. The process according to claim 9 in which the alkaline reagent used to remove the aliphatic acids from the heavy fraction is aqueous caustic soda.

11. The process according to claim 9 in which the percentage of aliphatic acids in the mixture of C3 and C4 hydrocarbons before treatment thereof is from 0.001% to 0.1% by weight.

12. The process of continuously obtaining a purified C3-C4 fraction from cracked hydrocarbon distillates which comprises: fractlonating a cracked distillate containing hydrocarbons with four carbonratoms together with lighter and heavier hydrocarbons, hydrogen sulde, aliphatic acids, and mercaptans to recover a composite fraction comprising predominantly C3 and C4 hydrocarbons together with the hydrogen 'sulfide and containing mercaptans and aliphatic acids; distilling said composite fraction to obtain a light fraction comprising said C3 hydrocarbons predominantly together with the hydrogen sultide, and a heavy fraction comprising said C4 hydrocarbons predominantly together with mercaptans and aliphatic acids; contacting a continuous stream of said light fraction in a contact zone with an alkaline reagent capable of absorbing HzS and liberating the same upon heating; removing contacted reagent from said contact zone; distilling H28 from said contacted alkaline reagent to regenerate the same; returning regenerated alkaline reagent to said contact zone; contacting said heavy fraction with an alkali solution to remove mercaptans and aliphatic acids; removing the purified heavy fraction from the alkali solution; and blending the thus treated light and heavy fractions to obtain a puried Ca-Cr mixture.

HERSCHEL Y. HYDE. CLARE KENNETH VILAND. LIONEL S. GALSTAUN. 

